There is presently an increasing demand for a means to quickly and accurately assay jewelry, coins and other precious metals in a retail environment.
Conventionally, such articles were appraised using the so-called “touchstone method”, an age-old technique that is relatively nondestructive to jewelry and offers quick results. Touchstone testing is based on the fact that 24k gold resists all but the strongest acids. However, since this method necessarily uses one or more acids, close attention to safety and careful procedures are a must.
In practicing this method, a portion of the article to be tested is rubbed against a black quartz test stone whereby an elemental deposit is obtained on the test stone which is then treated with one or more acids, by which the metallic composition of the test article is determined. To be properly performed, the touchstone method requires a certain level of skill and experience, and necessarily tends to mar the article being tested. Additionally, this test is not as effective on some alloys such as white gold (gold-palladium alloys) and platinum alloys which are significantly harder, nor to significantly softer materials such as fine gold, or certain other alloys containing more than 92% gold.